Cochlear implants in children: Reliability of computed tomography MICHAEL A. SEICSHNAYDRE, MD, MICHELE H. JOHNSON, MD, M. SUZANNE HASENSTAB, PhD, and GEORGE H. WILLIAMS, MD, FACS, Richmond, Virginia
Preoperative temporal bone computed tomography (CT) can demonstrate anatomic details relevant to surgical management and is therefore essential in the presurgical evaluation of patients receiving cochlear implants. The purpose of this study was to evaluate preoperative CTstudies and compare them to surgical findings in 34 children who received the Nucleus mUltichannel cochlear implant. The focus of this report is to discuss the dependability of CT scans in predicting surgical findings at the time of cochlear implantation. Results indicate that agreement of CT interpretations with surgical findings is partially related to the etiology of hearing loss and the experience of the surgeon and neuroradiologist. Advantages and limitations of the CT scans in predicting surgical findings are discussed. (OTOLARYNGOL HEAD NECK SURG 1992;107:410.)
A
common cause of profound sensorineural hearing loss in children is meningitis. Reports in the last 5 years indicate that the Nucleus multichannel cochlear implant has been successful in children with postmeningitic deafness, as well as in children with congenital hearing loss. Compromised patency of the basal tum of the cochlea and/ or ossification surrounding the round window are frequently encountered at surgery in postmeningitic children. The surgical approach in cochlear implantation consists of a limited mastoidectomy followed by opening of the facial recess (FR), entrance to the round window (RW), and placement of the electrode array into the scale tympani (ST) through the basal tum of the cochlea. The presence of ossification may inhibit insertion of the full electrode array of the device. Routine high-resolution computed tomography (CT) of the temporal bone is an integral part of the preoperative evaluation of children under consideration for
From the Departments of Otolaryngology-Head and Neck Surgery (Drs. Seicshnaydre, Hasenstab, and Williams) and Radiology (Dr. Johnson), Medical College of Virginia. Presented at the Annual Meeting of the American Academy of Otolaryngology-Head and Neck Surgery, Kansas City, Mo., Sept. 2226, 1991. Received for publication Dec. 6, 1991; revision received March 17, 1992; accepted March 30, 1992. Reprint requests: George H. Williams, MD, FACS, Department of Otolaryngology-Head and Neck Surgery, Medical College of Virginia, MCV P. O. Box 146, Richmond, VA 23298.
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Table 1. Total number of findings on CT scans by neuroradiologist Findings
No.
Normal scan Narrowed basal turn Bony lip at round window Ossified cochlea Widened cochlear aqueduct Bulbous lAC Right Mondini, left aplasia (Polytomography) CT not available
9 8 8 4 3 1 1 1 3 38
TOTAL*
*Four CT scans had two separate findings each.
cochlear implants and can provide important anatomic information. The capacity for CT to demonstrate cochlear patency, round window ossification, and bony congenital anomalies can help facilitate selection of implant site and may allow prediction of surgical difficulties during implantation, such as limited insertion of the electrode array. A CT taxonomy of cochlear patency at the basal tum, that includes a classification of normal, CI, C2, and C3 cochleas, has been described. 1 The Cl cochlea has slight indistinctness that correlates with fibrous tissue and beginning neoossification. The C2 cochlea shows segments of narrowing of basal tum, but with some areas of the scala tympani remaining open. The C3 cochlea has bony obliteration of the lumen.
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Cochlear implants in children: Reliabilily of CT 411
Fig. 1. Normal. A normal axial1.5-mm thick scan demonstrates the normal basal turn of the cochlea and the normal round window niche.
Table 2. Relationship of etiology of hearing loss to abnormal CT findings Etiology
No.
Congenital Meningitis Cytomegalovirus Brain anoxia
19 13 1
CT scans available for review
Abnormal findings
Percent abnormal
17 12 1 1 31
10 11 0 0 22
59% 92% 0% -.2% 71%
.r 34
TOTAL
Table 3. Agreement of CT findings with surgical findings
Finding Normal scan Narrowed basai turn Bony lip at round window Ossified cochlea Bulbous internal auditory canal Right Mondini, left aplasia CT not available TOTALS
No. of CT findings overall
CT findings confirmed at surgery
Abnormalities discovered at surgery ICT findings)
9
o
o
8 (7)*
5 4
1t 1t
4*
2t:j:
8 5 1 1 3 38 (34)§
16
Overall % true-positive 0% 71% 50% 80/100%* 100% 100% 47%
*Opposite side inserted. unable to confirm. tin one patient preoperative CT read as enlarged aqueduct only. operative findings were ossification, narrowing, and round window bony prominence. :j:CT scan in one patient read as narrowing only, operative findings included ossification. §Four scans had two abnormal findings.
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Fig. 2. Cochlear stenosis. Axial image through the right ear was obtained at 1.5-mm intervals. The scan demonstrates a tapered narrowing of the basal turn of the cochlea (arrowheads). More superiorly, a second area of focal stenosis is visible (arrow).
Table 4. Agreement of CT findings with surgical findings: Meningitis vs. nonmeningitis Abnormalities found at implantation
CT findings Findings
Normal scan Narrowed basal turn Bony lip at round window Ossified cochlea Bulbous internal auditory canal Right Mondini. left aplasia CT not available
Meningitis
Percent true-positive findings
Nonmeningilis
1
8
5 3
2 5
5 1
'Preoperative CT scan was used to select opposite ear for implantation.
PURPOSE
The purpose of this study was to compare preoperative CT interpretations with the surgical findings at the time of implantation for 34 children receiving the Nucleus multichannel device and evaluate the dependability of CT scans on the basis of the etiology of deafness. The aforementioned taxonomy was not used as categories were expanded to accommodate the objectives of the study. PROCEDURE
Between March 1987 and August 1991, CT scans for 34 children between the ages of 2.5 years and 15
years (mean age, 5.5 years), referred to the Medical College of Virginia (MCV) as possible candidates for cochlear implantation, were reviewed. All children received the Nucleus multichannel cochlear implant and the right ear of each child was implanted unless contraindicated by CT findings, physical disabilities, or asymmetry of hearing loss that indicated a left ear better choice. The current preoperative evaluation scan protocol at MCV for childhood deafness includes axial and coronal views of the temporal bone at 1 mm to 1.5 mm intervals using a high-resolution bone algorithm. Thirty-one of the 34 CT scans were reviewed by a single neurora-
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Cochlear implants in children: Reliabilily of CT 413
Fig. 3. Ossification at the round window niche. This axial section demonstrates a large chunk of bone overlying the round window niche on the right (arrow). A smaller ossification is present on the left. There is also bilateral narrowing of the basai turn of the cochlea, more prominently on the right than on the left,
Fig. 4. Cochlear ossifications. Axial image of the temporal bone was obtained at 1.5-mm intervals. There is mild narrowing of the basal turn of the cochlea on the right and several focal ossifications identified within the cochlea on the right (arrows).
diologist (three scans were unavailable). Tabulation was made of (I) the total number of abnormal findings on the CT scans; (2) agreement of CT findings with surgical findings; and (3) the relationship of etiology of hearing loss to abnormal CT findings. A CT finding is defined as positive if any feature demonstrated could prevent complete insertion of the
electrode array (i.e., cochlear stenosis) or could require modification of the surgical protocol in order to complete insertion (i.e., ossification). CT findings of a narrow basal turn of the cochlea or an ossified cochlea were confirmed at the time of surgery by unusual resistance to insertion or complete obstruction of the basal turn when there was an attempt
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to insert the electrode array. In these cases, insertion into the scala tympani often required additional drilling. Gross bony overgrowth of the round window at surgery confirmed CT findings for this condition. RESULTS
Cochlear Ossification
CT findings of an ossified cochlea showed the best agreement with surgical findings (Fig. 4). Of five scans read as an ossified cochlea, four were confirmed at surgery. The fifth patient also had a narrowed cochlear basal tum, so the CT scan was used in selection of the opposite ear for implantation (Table 4). Of the 26 CT scans read with no ossification, one ossified cochlea was found at surgery in a postmeningitic patient, creating a 4% false-negative rate.
Intepretation of the CT scans resulted in multiple findings (Table 1). Abnormal CT scans were found for 22 of 31 patients (71%) (Table 1). Nine scans (29%) were read as normal (Fig. 1). Categorization by etiology of hearing loss indicated that 92% of patients deafened by meningitis had abnormal CT scans, as compared to 59% of patients with congenital hearing loss (Table 2). Agreement of CT findings and specific findings at surgery showed true-positive rates of 100% in CT findings of ossified cochlea, bulbous internal auditory canal (lAC), and Mondini deformity (Table 3).
One of three patients with CT findings of a widened cochlear aqueduct was found to have a perilymph gusher at surgery. A perilymph gusher was also encountered in a single patient with a bulbous internal auditory canal.
Narrow Basal Turn of the Cochlea
Etiology
The preoperative CT reading of a narrow cochlear basal tum in one patient led to selection of the opposite ear for implantation. Five of seven patients with preoperative CT findings of a narrow basal tum (Fig. 2) were confirmed at surgery, creating a true-positive rate of 71%. All five of these patients had a hearing loss as a result of meningitis, achieving a true-positive rate of 100% in postmeningitic patients with this CT finding (Table 4). Two patients with congenital hearing loss had preoperative CT readings of narrowing, but revealed no such evidence at the time of implantation, thus creating a 2 of 7 (29%) false-positive rate. One postmeningitic patient was found to have a narrow basal tum at implantation, although the CT indicated only a widened cochlear aqueduct. Narrowing at the cochlear basal tum indicated on preoperative CT scans seems reliable only in patients with hearing loss as a result of meningitis.
Agreement of preoperative CT scans and surgical findings was also evaluated on the basis of etiology of hearing loss (Table 4). Hearing loss caused by meningitis resulted in a much higher rate of true-positive CT scans in all categories of preoperative CT findings. A true-positive CT scan rate of 100% was achieved for CT findings of narrow cochlear basal tum, bony lip at the round window, ossified cochlea, widened cochlear aqueduct, and bulbous lAC for postmeningitic patients (Table 4). Overall agreement of preoperative CT scan resulted in a false-positive rate of only 8% for postmeningitic patients as compared with 35% for congenitally deafened patients.
Bony Lip at the Round Window
Three of four (75%) true-positive scans indicating bony lip at the round window were for cases of postmeningitic hearing loss and one was for a congenital loss (Fig. 3). All four false-positive scans were in patients with congenital deafness. True-positive rates were 100% in postmeningitic patients and 20% in patients with congenital deafness (Table 4). One postmeningitic patient with a negative CT scan presented the surgical finding of a large bony lip at the round window that required extensive drilling. The neuroradiologist reported this scan was a limited quality scan obtained at 3-mm thick slices at an atypical angle.
Perilymph Gusher
DISCUSSION
High-resolution CT scans of the temporal bone have been shown to be helpful in considering children as candidates for multichannel cochlear implants.?" In cases of postmeningitic hearing loss, reports indicate CT scan evidence of bone formation in up to 73% of patients implanted.' Twenty-two of 31 scans reviewed in this study (71%) were noted to have some abnormality (Table 1). This confirms the importance of CT scans for multichannel cochlear implant patients. Most abnormalities in this series were related to reactive cochlear changes, such as cochlear ossification. Indications of congenital abnormalities were helpful in preoperative planning. Cochlear Patency
Luxford and House," in a series of more than 100 children who received cochlear implants, report pre-
Volume 107 Number 3 September 1992
operative CT indications of ossification in 21. 3%, with surgical findings in 33.9% of cases. Additionally, 37% of the patients appeared to have round window ossification on CT scan, with a 51.2% of patients manifesting round window ossifications at surgery. Luxford and House report that 64% of these children were deafened by meningitis. Jackler et al.' reported that patients with postmeningitic deafness had abnormal CT scans in 63% of cases; all were found to have some degree of ossification. The current study found 92% abnormal scans, with 83% indicating some degree of ossification at surgery. This created a false-positive rate of 8% and a false-negative rate of 8%. Jackler et aI. 3 also report an overall truepositive rate of 100% and a false-negative rate of 46% for agreement of preoperative CT findings of narrow cochlear basal tum or obliteration and findings at surgery. Current findings of a true-positive rate of 100% and a false-negative rate of 14% are in agreement with Jackler et aI.' s results (Table 4). Present findings support current literature suggesting that preoperative CT scanning is accurate (high rate of true-positive scans) in defining narrow cochlear basal turns and ossified cochleas. The round window and basal tum of the cochlea are described as the most common areas of ossification after meningitis. 1 The surgeon must be prepared to drill to provide a channel for the long multi-electrode array. Luxford and House' reported drilling 6 to 7 mm into the scali tympani to find a good channel for insertion. In this series, there is an overall 50% true-positive rate in predicting ossification at the round window (Table 3). When considering postmeningitic patients, a 100% true-positive rate is noted. An 80% false-positive rate is noted in patients with congenital hearing loss. Preoperative CT scans are helpful in predicting the possible need for additional drilling of the scale tympani in postmeningitic hearing loss. Cochlear Deformities
It has been suggested that a perilymph gusher may result from abnormal patency of the cochlear aqueduct. 6 Three patients had preoperative CT scans interpreted as widened cochlear aqueduct. Only one perilymph gusher was encountered at surgery. A perilymph gusher was found in another patient whose preoperative CT scan demonstrated a bulbous internal auditory canal. CT indications of a widened cochlear aqueduct or bulbous lAC should be reported to the surgeon before implantation. A narrowed lAC is also significant. Although not encountered in this patient population, the
Cochlear implants in children: Reliability of CT 415
narrow lAC on CT suggests aplasia of the auditoryvestibular nerve, and thus represents a contraindication to cochlear implantation. 7 Silverstein et aI. 8 first reported multichannel implantation of Mondini deformities with good audiologic results. One scan in this series indicated a Mondini deformity on one side and a cochlear aplasia on the other (Fig. 5). Implantation on the Mondini side produced good audiologic results. Although profuse perilymph flow in all patients with this abnormality has been reported, it was not found in this patient." A Mondini deformity should not be considered a contraindication to cochlear implantation. Selection of the Better Ear
As in the above case, preoperative CT scans can be helpful in selection of the better ear to implant.' Differences between ears may include the degree of ossification or stenosis in the ears of postmeningitic patients.' In one postmeningitic patient in this study, the preoperative CT scan also indicated unilateral narrowing of the cochlear basal tum that influenced the side selected for implantation. Dependability of CT Scans
Weit et aI. report limited predictive ability of the CT in cases with a history of meningitis. 10 In contrast, this study finds CT to be most reliable in patients with postmeningitic hearing loss. Meningitis is the most common cause of cochlear ossification in children. Eisenberger et aI. II reported that 20 of 25 children deafened by meningitis had some degree of ossification at the round window or scala tympani that required drilling for insertion. In the current study, postmeningitic changes were categorized as round window bony ridges, narrow cochlear basal tum, and ossification in the basal tum. Although preoperative CT scans in this and other studies show high truepositive rates in patients with meningitis, many also show a significant false-negative rate. This suggests that CT findings of ossification, narrow basal tum, or round window bony changes in patients deafened by meningitis generally agree with operative findings. However, false-negative CT scans are not predictors of the occurrence of operative findings. The surgeon must be prepared for the possibility of further surgical modification when CT scans read normal cochlear patency in postmeningitic patients. Experience of Surgeon and Neuroradiologist
The senior author implanted all patients in the present study. Although not quantified, experience gained from
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et 01.
Fig. 5. Congenital cochlear anomalies. Notice the asymmetry of the temporal bones in this patient. The right cochlea demonstrates the lack of cochlear turns lypical of a Mondini deformily (arrows). The left cochlea is aplastic; the temporal bone is reduced in thickness, with solid bone in the place of the absent cochlea.
repeated implantations and CT comparisons improves the ability of the surgeon and the neuroradiologist to predict surgical findings when studying preoperative CT scans. Review of CT scans by the surgeon with the neuroradiologist both before and after the surgical procedure provides the radiologic/pathologic correlation and experience necessary for preoperative diagnostic accuracy. CT interpretation by a neuroradiologist skilled in the radiographic representation of temporal bone anatomy and familiar with the cochlear implant surgical procedure is crucial to the dependability of preoperative CT. CT scans from outside hospitals should be reviewed by the operating hospital's neuroradiologist. High-resolution axial CT scans of1.5-mm cuts should give two to three images of the cochlea, providing adequate visualization of the round window and basal tum. All but two of our cases received 1.5-mm high-resolution CT scans. It has been suggested that l-rnrn cuts provide the best view of the pertinent structures in the temporal bone." CONCLUSIONS
Results of this study suggest: (1) CT scans are most predictive of operative findings in patients with postmeningitic hearing loss; (2) positive CT findings of
bony overgrowth at the round window or basal tum narrowing are most likely to agree with surgical findings of ossification; (3) CT scans may be helpful in predicting the number of possible inserted electrodes; (4) CT scans are helpful in selection of the better ear to implant; (5) a negative CT in a postmeningitic patient .does not exclude fibro-ossification with certainty; (6) the surgeon should always be prepared to modify surgical protocol, regardless of CT indications; (7) interpretation of high-resolution CT scans by an experienced neuroradiologist with knowledge of temporal bone anatomy and the cochlear implant surgical procedure is crucial for reliable readings; and (8) experience gained in this series of implants has undoubtedly improved the neuroradiologist's and the operating surgeon's ability to critically interpret CT scans preoperatively. We wish to thank Ms. Betty Muse for her efforts in preparation of this manuscript. REFERENCES
I. Balkany T, Driesbach J. Workshop: Surgical anatomy and radiographic imaging of cochlear implant surgery. Am J Otol 1987;8: 195-200. 2. Balkany T, Gantz B, Nadal J. Multichannel cochlear implants in partially ossified cochleas. Ann Otol Rhinal Laryngol 1988;97:3-7.
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3. Jackler RK, Luxford WM, Schindler RA, McKerrow WS. Cochlear patency problems in cochlear implantation. Laryngoscope 1987;97:801-5. 4. Phelps PD, Annis JAD, Robinson PJ. Imaging for cochlear implants. Br J Radiol 1990;63:512-6. 5. Luxford WM, House WF. Cochlear implants in children: medical and surgical considerations. Ear Hear 1985;6:20S-23S. 6. Farrior B, Endicott IN. Congenital mixed deafness: cerebral spinal fluid otorrhea: ablation of the aqueduct of the cochlea. Laryngoscope 1971;81:684-9. 7. Shelton C, Luxford WM, Tonokawa LL, Lo WM, House WF. The narrow internal auditory canal in children: a contraindication to cochlear implants. OTOLARYNGOL HEAD NECK SURG 1989;100:227-31.
Cochlear implants in children: Reliability of CT 417
8. Silverstein H, Smouha E, Morgan N. Multichannel cochlear implantation in a patient with bilateral Mondini deformities. Am J Otol 1988;9:451-5. 9. Miyamoto RT, McConkey AJ, Myres WA, Pope ML. Cochlear implantation in the Mondini inner ear malformation. Am J Otol 1986;7:258-61. 10. Wiet RF, Pyle GM, O'Connor CA, Russell E, Schramm DR. Computed tomography: how accurate a predicator for cochlear implantation. Laryngoscope 1990;100:687-92. II. Eisenberger LS, Luxford WM, Becker TS, House WF. Electrical stimulation of the auditory system in children deafened by meningitis. OTOLARYNGOL HEAD NECK SURG 1984;92:700-5.
Preoperative temporal bone computed tomography (CT) can demonstrate anatomic details relevant to surgical management and is therefore essential in the presurgical evaluation of patients receiving cochlear implants. The purpose of this study was to evaluate preoperative CTstudies and compare them to surgical findings in 34 children who received the Nucleus mUltichannel cochlear implant. The focus of this report is to discuss the dependability of CT scans in predicting surgical findings at the time of cochlear implantation. Results indicate that agreement of CT interpretations with surgical findings is partially related to the etiology of hearing loss and the experience of the surgeon and neuroradiologist. Advantages and limitations of the CT scans in predicting surgical findings are discussed. (OTOLARYNGOL HEAD NECK SURG 1992;107:410.)
A
common cause of profound sensorineural hearing loss in children is meningitis. Reports in the last 5 years indicate that the Nucleus multichannel cochlear implant has been successful in children with postmeningitic deafness, as well as in children with congenital hearing loss. Compromised patency of the basal tum of the cochlea and/ or ossification surrounding the round window are frequently encountered at surgery in postmeningitic children. The surgical approach in cochlear implantation consists of a limited mastoidectomy followed by opening of the facial recess (FR), entrance to the round window (RW), and placement of the electrode array into the scale tympani (ST) through the basal tum of the cochlea. The presence of ossification may inhibit insertion of the full electrode array of the device. Routine high-resolution computed tomography (CT) of the temporal bone is an integral part of the preoperative evaluation of children under consideration for
From the Departments of Otolaryngology-Head and Neck Surgery (Drs. Seicshnaydre, Hasenstab, and Williams) and Radiology (Dr. Johnson), Medical College of Virginia. Presented at the Annual Meeting of the American Academy of Otolaryngology-Head and Neck Surgery, Kansas City, Mo., Sept. 2226, 1991. Received for publication Dec. 6, 1991; revision received March 17, 1992; accepted March 30, 1992. Reprint requests: George H. Williams, MD, FACS, Department of Otolaryngology-Head and Neck Surgery, Medical College of Virginia, MCV P. O. Box 146, Richmond, VA 23298.
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Table 1. Total number of findings on CT scans by neuroradiologist Findings
No.
Normal scan Narrowed basal turn Bony lip at round window Ossified cochlea Widened cochlear aqueduct Bulbous lAC Right Mondini, left aplasia (Polytomography) CT not available
9 8 8 4 3 1 1 1 3 38
TOTAL*
*Four CT scans had two separate findings each.
cochlear implants and can provide important anatomic information. The capacity for CT to demonstrate cochlear patency, round window ossification, and bony congenital anomalies can help facilitate selection of implant site and may allow prediction of surgical difficulties during implantation, such as limited insertion of the electrode array. A CT taxonomy of cochlear patency at the basal tum, that includes a classification of normal, CI, C2, and C3 cochleas, has been described. 1 The Cl cochlea has slight indistinctness that correlates with fibrous tissue and beginning neoossification. The C2 cochlea shows segments of narrowing of basal tum, but with some areas of the scala tympani remaining open. The C3 cochlea has bony obliteration of the lumen.
Volume 107 Number 3 September 1992
Cochlear implants in children: Reliabilily of CT 411
Fig. 1. Normal. A normal axial1.5-mm thick scan demonstrates the normal basal turn of the cochlea and the normal round window niche.
Table 2. Relationship of etiology of hearing loss to abnormal CT findings Etiology
No.
Congenital Meningitis Cytomegalovirus Brain anoxia
19 13 1
CT scans available for review
Abnormal findings
Percent abnormal
17 12 1 1 31
10 11 0 0 22
59% 92% 0% -.2% 71%
.r 34
TOTAL
Table 3. Agreement of CT findings with surgical findings
Finding Normal scan Narrowed basai turn Bony lip at round window Ossified cochlea Bulbous internal auditory canal Right Mondini, left aplasia CT not available TOTALS
No. of CT findings overall
CT findings confirmed at surgery
Abnormalities discovered at surgery ICT findings)
9
o
o
8 (7)*
5 4
1t 1t
4*
2t:j:
8 5 1 1 3 38 (34)§
16
Overall % true-positive 0% 71% 50% 80/100%* 100% 100% 47%
*Opposite side inserted. unable to confirm. tin one patient preoperative CT read as enlarged aqueduct only. operative findings were ossification, narrowing, and round window bony prominence. :j:CT scan in one patient read as narrowing only, operative findings included ossification. §Four scans had two abnormal findings.
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Fig. 2. Cochlear stenosis. Axial image through the right ear was obtained at 1.5-mm intervals. The scan demonstrates a tapered narrowing of the basal turn of the cochlea (arrowheads). More superiorly, a second area of focal stenosis is visible (arrow).
Table 4. Agreement of CT findings with surgical findings: Meningitis vs. nonmeningitis Abnormalities found at implantation
CT findings Findings
Normal scan Narrowed basal turn Bony lip at round window Ossified cochlea Bulbous internal auditory canal Right Mondini. left aplasia CT not available
Meningitis
Percent true-positive findings
Nonmeningilis
1
8
5 3
2 5
5 1
'Preoperative CT scan was used to select opposite ear for implantation.
PURPOSE
The purpose of this study was to compare preoperative CT interpretations with the surgical findings at the time of implantation for 34 children receiving the Nucleus multichannel device and evaluate the dependability of CT scans on the basis of the etiology of deafness. The aforementioned taxonomy was not used as categories were expanded to accommodate the objectives of the study. PROCEDURE
Between March 1987 and August 1991, CT scans for 34 children between the ages of 2.5 years and 15
years (mean age, 5.5 years), referred to the Medical College of Virginia (MCV) as possible candidates for cochlear implantation, were reviewed. All children received the Nucleus multichannel cochlear implant and the right ear of each child was implanted unless contraindicated by CT findings, physical disabilities, or asymmetry of hearing loss that indicated a left ear better choice. The current preoperative evaluation scan protocol at MCV for childhood deafness includes axial and coronal views of the temporal bone at 1 mm to 1.5 mm intervals using a high-resolution bone algorithm. Thirty-one of the 34 CT scans were reviewed by a single neurora-
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Cochlear implants in children: Reliabilily of CT 413
Fig. 3. Ossification at the round window niche. This axial section demonstrates a large chunk of bone overlying the round window niche on the right (arrow). A smaller ossification is present on the left. There is also bilateral narrowing of the basai turn of the cochlea, more prominently on the right than on the left,
Fig. 4. Cochlear ossifications. Axial image of the temporal bone was obtained at 1.5-mm intervals. There is mild narrowing of the basal turn of the cochlea on the right and several focal ossifications identified within the cochlea on the right (arrows).
diologist (three scans were unavailable). Tabulation was made of (I) the total number of abnormal findings on the CT scans; (2) agreement of CT findings with surgical findings; and (3) the relationship of etiology of hearing loss to abnormal CT findings. A CT finding is defined as positive if any feature demonstrated could prevent complete insertion of the
electrode array (i.e., cochlear stenosis) or could require modification of the surgical protocol in order to complete insertion (i.e., ossification). CT findings of a narrow basal turn of the cochlea or an ossified cochlea were confirmed at the time of surgery by unusual resistance to insertion or complete obstruction of the basal turn when there was an attempt
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to insert the electrode array. In these cases, insertion into the scala tympani often required additional drilling. Gross bony overgrowth of the round window at surgery confirmed CT findings for this condition. RESULTS
Cochlear Ossification
CT findings of an ossified cochlea showed the best agreement with surgical findings (Fig. 4). Of five scans read as an ossified cochlea, four were confirmed at surgery. The fifth patient also had a narrowed cochlear basal tum, so the CT scan was used in selection of the opposite ear for implantation (Table 4). Of the 26 CT scans read with no ossification, one ossified cochlea was found at surgery in a postmeningitic patient, creating a 4% false-negative rate.
Intepretation of the CT scans resulted in multiple findings (Table 1). Abnormal CT scans were found for 22 of 31 patients (71%) (Table 1). Nine scans (29%) were read as normal (Fig. 1). Categorization by etiology of hearing loss indicated that 92% of patients deafened by meningitis had abnormal CT scans, as compared to 59% of patients with congenital hearing loss (Table 2). Agreement of CT findings and specific findings at surgery showed true-positive rates of 100% in CT findings of ossified cochlea, bulbous internal auditory canal (lAC), and Mondini deformity (Table 3).
One of three patients with CT findings of a widened cochlear aqueduct was found to have a perilymph gusher at surgery. A perilymph gusher was also encountered in a single patient with a bulbous internal auditory canal.
Narrow Basal Turn of the Cochlea
Etiology
The preoperative CT reading of a narrow cochlear basal tum in one patient led to selection of the opposite ear for implantation. Five of seven patients with preoperative CT findings of a narrow basal tum (Fig. 2) were confirmed at surgery, creating a true-positive rate of 71%. All five of these patients had a hearing loss as a result of meningitis, achieving a true-positive rate of 100% in postmeningitic patients with this CT finding (Table 4). Two patients with congenital hearing loss had preoperative CT readings of narrowing, but revealed no such evidence at the time of implantation, thus creating a 2 of 7 (29%) false-positive rate. One postmeningitic patient was found to have a narrow basal tum at implantation, although the CT indicated only a widened cochlear aqueduct. Narrowing at the cochlear basal tum indicated on preoperative CT scans seems reliable only in patients with hearing loss as a result of meningitis.
Agreement of preoperative CT scans and surgical findings was also evaluated on the basis of etiology of hearing loss (Table 4). Hearing loss caused by meningitis resulted in a much higher rate of true-positive CT scans in all categories of preoperative CT findings. A true-positive CT scan rate of 100% was achieved for CT findings of narrow cochlear basal tum, bony lip at the round window, ossified cochlea, widened cochlear aqueduct, and bulbous lAC for postmeningitic patients (Table 4). Overall agreement of preoperative CT scan resulted in a false-positive rate of only 8% for postmeningitic patients as compared with 35% for congenitally deafened patients.
Bony Lip at the Round Window
Three of four (75%) true-positive scans indicating bony lip at the round window were for cases of postmeningitic hearing loss and one was for a congenital loss (Fig. 3). All four false-positive scans were in patients with congenital deafness. True-positive rates were 100% in postmeningitic patients and 20% in patients with congenital deafness (Table 4). One postmeningitic patient with a negative CT scan presented the surgical finding of a large bony lip at the round window that required extensive drilling. The neuroradiologist reported this scan was a limited quality scan obtained at 3-mm thick slices at an atypical angle.
Perilymph Gusher
DISCUSSION
High-resolution CT scans of the temporal bone have been shown to be helpful in considering children as candidates for multichannel cochlear implants.?" In cases of postmeningitic hearing loss, reports indicate CT scan evidence of bone formation in up to 73% of patients implanted.' Twenty-two of 31 scans reviewed in this study (71%) were noted to have some abnormality (Table 1). This confirms the importance of CT scans for multichannel cochlear implant patients. Most abnormalities in this series were related to reactive cochlear changes, such as cochlear ossification. Indications of congenital abnormalities were helpful in preoperative planning. Cochlear Patency
Luxford and House," in a series of more than 100 children who received cochlear implants, report pre-
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operative CT indications of ossification in 21. 3%, with surgical findings in 33.9% of cases. Additionally, 37% of the patients appeared to have round window ossification on CT scan, with a 51.2% of patients manifesting round window ossifications at surgery. Luxford and House report that 64% of these children were deafened by meningitis. Jackler et al.' reported that patients with postmeningitic deafness had abnormal CT scans in 63% of cases; all were found to have some degree of ossification. The current study found 92% abnormal scans, with 83% indicating some degree of ossification at surgery. This created a false-positive rate of 8% and a false-negative rate of 8%. Jackler et aI. 3 also report an overall truepositive rate of 100% and a false-negative rate of 46% for agreement of preoperative CT findings of narrow cochlear basal tum or obliteration and findings at surgery. Current findings of a true-positive rate of 100% and a false-negative rate of 14% are in agreement with Jackler et aI.' s results (Table 4). Present findings support current literature suggesting that preoperative CT scanning is accurate (high rate of true-positive scans) in defining narrow cochlear basal turns and ossified cochleas. The round window and basal tum of the cochlea are described as the most common areas of ossification after meningitis. 1 The surgeon must be prepared to drill to provide a channel for the long multi-electrode array. Luxford and House' reported drilling 6 to 7 mm into the scali tympani to find a good channel for insertion. In this series, there is an overall 50% true-positive rate in predicting ossification at the round window (Table 3). When considering postmeningitic patients, a 100% true-positive rate is noted. An 80% false-positive rate is noted in patients with congenital hearing loss. Preoperative CT scans are helpful in predicting the possible need for additional drilling of the scale tympani in postmeningitic hearing loss. Cochlear Deformities
It has been suggested that a perilymph gusher may result from abnormal patency of the cochlear aqueduct. 6 Three patients had preoperative CT scans interpreted as widened cochlear aqueduct. Only one perilymph gusher was encountered at surgery. A perilymph gusher was found in another patient whose preoperative CT scan demonstrated a bulbous internal auditory canal. CT indications of a widened cochlear aqueduct or bulbous lAC should be reported to the surgeon before implantation. A narrowed lAC is also significant. Although not encountered in this patient population, the
Cochlear implants in children: Reliability of CT 415
narrow lAC on CT suggests aplasia of the auditoryvestibular nerve, and thus represents a contraindication to cochlear implantation. 7 Silverstein et aI. 8 first reported multichannel implantation of Mondini deformities with good audiologic results. One scan in this series indicated a Mondini deformity on one side and a cochlear aplasia on the other (Fig. 5). Implantation on the Mondini side produced good audiologic results. Although profuse perilymph flow in all patients with this abnormality has been reported, it was not found in this patient." A Mondini deformity should not be considered a contraindication to cochlear implantation. Selection of the Better Ear
As in the above case, preoperative CT scans can be helpful in selection of the better ear to implant.' Differences between ears may include the degree of ossification or stenosis in the ears of postmeningitic patients.' In one postmeningitic patient in this study, the preoperative CT scan also indicated unilateral narrowing of the cochlear basal tum that influenced the side selected for implantation. Dependability of CT Scans
Weit et aI. report limited predictive ability of the CT in cases with a history of meningitis. 10 In contrast, this study finds CT to be most reliable in patients with postmeningitic hearing loss. Meningitis is the most common cause of cochlear ossification in children. Eisenberger et aI. II reported that 20 of 25 children deafened by meningitis had some degree of ossification at the round window or scala tympani that required drilling for insertion. In the current study, postmeningitic changes were categorized as round window bony ridges, narrow cochlear basal tum, and ossification in the basal tum. Although preoperative CT scans in this and other studies show high truepositive rates in patients with meningitis, many also show a significant false-negative rate. This suggests that CT findings of ossification, narrow basal tum, or round window bony changes in patients deafened by meningitis generally agree with operative findings. However, false-negative CT scans are not predictors of the occurrence of operative findings. The surgeon must be prepared for the possibility of further surgical modification when CT scans read normal cochlear patency in postmeningitic patients. Experience of Surgeon and Neuroradiologist
The senior author implanted all patients in the present study. Although not quantified, experience gained from
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OlolaryngologyHead and Neck Surgery
et 01.
Fig. 5. Congenital cochlear anomalies. Notice the asymmetry of the temporal bones in this patient. The right cochlea demonstrates the lack of cochlear turns lypical of a Mondini deformily (arrows). The left cochlea is aplastic; the temporal bone is reduced in thickness, with solid bone in the place of the absent cochlea.
repeated implantations and CT comparisons improves the ability of the surgeon and the neuroradiologist to predict surgical findings when studying preoperative CT scans. Review of CT scans by the surgeon with the neuroradiologist both before and after the surgical procedure provides the radiologic/pathologic correlation and experience necessary for preoperative diagnostic accuracy. CT interpretation by a neuroradiologist skilled in the radiographic representation of temporal bone anatomy and familiar with the cochlear implant surgical procedure is crucial to the dependability of preoperative CT. CT scans from outside hospitals should be reviewed by the operating hospital's neuroradiologist. High-resolution axial CT scans of1.5-mm cuts should give two to three images of the cochlea, providing adequate visualization of the round window and basal tum. All but two of our cases received 1.5-mm high-resolution CT scans. It has been suggested that l-rnrn cuts provide the best view of the pertinent structures in the temporal bone." CONCLUSIONS
Results of this study suggest: (1) CT scans are most predictive of operative findings in patients with postmeningitic hearing loss; (2) positive CT findings of
bony overgrowth at the round window or basal tum narrowing are most likely to agree with surgical findings of ossification; (3) CT scans may be helpful in predicting the number of possible inserted electrodes; (4) CT scans are helpful in selection of the better ear to implant; (5) a negative CT in a postmeningitic patient .does not exclude fibro-ossification with certainty; (6) the surgeon should always be prepared to modify surgical protocol, regardless of CT indications; (7) interpretation of high-resolution CT scans by an experienced neuroradiologist with knowledge of temporal bone anatomy and the cochlear implant surgical procedure is crucial for reliable readings; and (8) experience gained in this series of implants has undoubtedly improved the neuroradiologist's and the operating surgeon's ability to critically interpret CT scans preoperatively. We wish to thank Ms. Betty Muse for her efforts in preparation of this manuscript. REFERENCES
I. Balkany T, Driesbach J. Workshop: Surgical anatomy and radiographic imaging of cochlear implant surgery. Am J Otol 1987;8: 195-200. 2. Balkany T, Gantz B, Nadal J. Multichannel cochlear implants in partially ossified cochleas. Ann Otol Rhinal Laryngol 1988;97:3-7.
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3. Jackler RK, Luxford WM, Schindler RA, McKerrow WS. Cochlear patency problems in cochlear implantation. Laryngoscope 1987;97:801-5. 4. Phelps PD, Annis JAD, Robinson PJ. Imaging for cochlear implants. Br J Radiol 1990;63:512-6. 5. Luxford WM, House WF. Cochlear implants in children: medical and surgical considerations. Ear Hear 1985;6:20S-23S. 6. Farrior B, Endicott IN. Congenital mixed deafness: cerebral spinal fluid otorrhea: ablation of the aqueduct of the cochlea. Laryngoscope 1971;81:684-9. 7. Shelton C, Luxford WM, Tonokawa LL, Lo WM, House WF. The narrow internal auditory canal in children: a contraindication to cochlear implants. OTOLARYNGOL HEAD NECK SURG 1989;100:227-31.
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8. Silverstein H, Smouha E, Morgan N. Multichannel cochlear implantation in a patient with bilateral Mondini deformities. Am J Otol 1988;9:451-5. 9. Miyamoto RT, McConkey AJ, Myres WA, Pope ML. Cochlear implantation in the Mondini inner ear malformation. Am J Otol 1986;7:258-61. 10. Wiet RF, Pyle GM, O'Connor CA, Russell E, Schramm DR. Computed tomography: how accurate a predicator for cochlear implantation. Laryngoscope 1990;100:687-92. II. Eisenberger LS, Luxford WM, Becker TS, House WF. Electrical stimulation of the auditory system in children deafened by meningitis. OTOLARYNGOL HEAD NECK SURG 1984;92:700-5.
